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https://issues.apache.org/jira/browse/LUCENE-5591?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13965403#comment-13965403
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Michael McCandless commented on LUCENE-5591:
--------------------------------------------

+1, good catch.

I think that guesstimate is a good start?  It likely wildly over-estimates 
though, since in-RAM structures are usually much more costly than the on-disk 
format, maybe try it out and see how much it over-estimates?

> ReaderAndUpdates should create a proper IOContext when writing DV updates
> -------------------------------------------------------------------------
>
>                 Key: LUCENE-5591
>                 URL: https://issues.apache.org/jira/browse/LUCENE-5591
>             Project: Lucene - Core
>          Issue Type: Improvement
>            Reporter: Shai Erera
>
> Today we pass IOContext.DEFAULT. If DV updates are used in conjunction w/ 
> NRTCachingDirectory, it means the latter will attempt to write the entire DV 
> field in its RAMDirectory, which could lead to OOM.
> Would be good if we can build our own FlushInfo, estimating the number of 
> bytes we're about to write. I didn't see off hand a quick way to guesstimate 
> that - I thought to use the current DV's sizeInBytes as an approximation, but 
> I don't see a way to get it, not a direct way at least.
> Maybe we can use the size of the in-memory updates to guesstimate that 
> amount? Something like {{sizeOfInMemUpdates * (maxDoc/numUpdatedDocs)}}? Is 
> it a too wild guess?



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